A breakdown electric field intensity of the third-generation semiconductor gallium nitride (GaN) material is much higher than that of the first-generation semiconductor silicon (Si) material or the second-generation semiconductor gallium arsenide (GaAs) material, thus electronic devices based on gallium nitride are capable of withstanding higher operating voltages. In addition, gallium nitride can form a heterojunction together with other III-N compound semiconductors, and has a two-dimensional electron gas (2DEG) channel with high concentration. Therefore, GaN High Electron Mobility Transistors (HEMTs) are much suitable for the manufacture of high-power electronic devices due to their characteristics of high voltages and high currents, which brings a good prospect.
The HEMT device is a kind of planar channel field effect transistors in which most of the electric field lines are gathered at an edge of a gate electrode adjacent to a drain electrode so as to form a high electric field spike. As a voltage applied between the gate electrode and the drain electrode increases, the electric field intensity at this position will increase rapidly, so that the gate leaking current is increased. Such a high electric field in a local region will easily cause the device to fail due to avalanche breakdown, thus reducing the breakdown voltage of the device. Meanwhile, with increase of working time, the high electric field will cause degradation or denaturation of a surface dielectric layer or a semiconductor material layer, thus affecting reliability of the device and reducing lifetime of the device.
A field plate is usually placed at a side of the gate electrode near the drain electrode in the prior art, so as to reduce the strong electric field in the vicinity of the gate electrode of the device, thereby improving the breakdown voltage of the device and obtaining excellent reliability. FIG. 1 illustrates a field plate power device according to the prior art. As shown in FIG. 1, a field plate power device comprises: a substrate 101; a nucleation layer 102, a buffer layer 103, a channel layer 104 and a barrier layer 105 sequentially stacked on the substrate 101; a source electrode 106, a drain electrode 107, and a gate electrode 108 between the source electrode 106 and the drain electrode 107 disposed on the barrier layer 105; a dielectric layer 109 disposed on the gate electrode 108, a part of the barrier layer 105 between the gate electrode 108 and the source electrode 106 and another part of the barrier layer 105 between the gate electrode 108 and the drain electrode 107; and a metal field plate 110 disposed on the dielectric layer 109.
A bottom of the metal field plate 110 is substantially parallel to the barrier layer 105. The metal field plate 110 is connected to the source electrode 106 or the gate electrode 108, and generates an additional potential in the gate-drain region. Therefore, the electric field spike near an edge of the gate electrode 108 adjacent to the drain electrode 107 can be suppressed, and the breakdown voltage of the device and the reliability of the device can be improved. However, since the bottom of the field plate is substantially parallel to the surface of the barrier layer 105, although the electric field spike near the edge of the gate electrode 108 can be reduced, a new low electric field spike will be formed near an end of the field plate. The new electric field spike will increase as the length of the field plate increases, which will lead to breakdown or failure in the region near the end of the field plate. Accordingly, the problem of breakdown of the device has not been solved fundamentally, and the risk of breakdown is just transferred to another region. In addition, if a field plate is too long, a relatively high parasitic capacitance will be generated, which will affect high frequency power characteristics of the device.
In order to address such an issue, a field plate structure in which a plurality of layers, e.g., three layers, are stacked is utilized in the prior art, so as to form a gradient distribution of electric potential. Such a field plate structure with gradient distribution should be made by multi-step lithography, dielectric deposition, metal deposition and other processes, the production process is complicated and the manufacturing cost of the device is increased. Moreover, for such a multi-leveled field plate, it is difficult to evenly distribute the electric field on the surface of the device, thus an excellent device performance is difficult to be obtained.